Sleep may be characterized by four stages, stage one sleep, stage two sleep, stage three sleep, and rapid eye movements (REM) stage sleep. The four stages may form a sleep cycle. Typically in a sleep cycle, a person experiences the four stages in sequence (e.g., stage one sleep→stage two sleep→stage three sleep→REM stage sleep). A person may experience several sleep cycles during a sleep period (e.g., during a night). The number of sleep cycles a person experiences per sleep period depends on the age of the person, duration of the sleep period, and other factors.
Neurological disorders (e.g., a seizure disorder or depression) may interfere with a person's sleep quality. For example, a patient with a seizure disorder may experience seizures during sleep. The seizures experienced during sleep may affect the patient's sleep quality. When the patient has a seizure during sleep, the seizure may awaken the patient prematurely from a particular sleep cycle. The patient does not finish the particular sleep cycle and has to fall back asleep to start another sleep cycle. Thus, the patient experiences sleep deprivation.